Pencil beam scanning (“PBS”) techniques have gained more popularity in proton therapy centers because PBS has reduced neutron dose; no need for apertures and compensators; and intrinsically enables intensity modulated proton therapy (“IMPT”). Due to technical challenges, current clinical PBS systems cannot provide protons with energies below 70 MeV. This limits the ability to treat shallow tumors (e.g., tumors at a depth less than 4 cm) unless a range shifter (“RS”) is added at the end of the nozzle.
The spot sizes of proton beamlets increase when a RS is used, and these increases in spot size are more pronounced for low proton energies. This increase in spot size is due to the scattering of protons from the RS and the large air gap between the RS and isocenter. Additionally, the RS creates low-signal tails in the beam profile due to large-angle multiple Coulomb scattering and nuclear interactions. These low-signal tails can extend laterally more than 10 cm from the spot center. Clinically these low dose tails manifest themselves in an increase of output with field sizes, a phenomenon known as the field size effect. To address this clinically-relevant effect, the treatment planning system (“TPS”) can use two Gaussian distributions to model the proton fluence: one for the primary fluence and the second for the tails. The modeling process for a RS in a TPS is further complicated by spot size enlargement and the associated low-dose tails.
The process of commissioning a proton therapy system, or performing routine quality assurance on such a system, can also be time consuming. Currently, field size factors are computed based on radiation measurements obtained by scanning a field-of-view using different beam deliveries. That is, the beam is scanned across the entire field and radiation measurements are recorded before the beam is moved over the next field. It would be beneficial to provide a method for measuring field size factor in multiple different field sizes using a more efficient process.